Drastic improvements in transmission speed and transmission quality in recent years have led digital radio communications to occupy an important place in the communication field. On the other hand, since radio communications use radio space that is a public asset, there is a basic defect, that a third party may be able to receive, from the standpoint of confidentiality. Namely, there is always a constant risk of communication contents being intercepted by a third party and information leaked out.
Thus, in conventional radio communications, techniques such as encryption of confidential information are used to prevent a third party from understanding the contents of the hidden information even if transmitting data is intercepted by the third party. Encryption technique has been studied and applied in various fields. This is because encryption has the advantage of enabling constant security to be assured without changing a radio communication system.
However, through the process of information encryption, there is such a problem that information can be decrypted comparatively easily if the code and/or procedure for encryption are known. With the current state prevalent of high-speed computers, in particular, security can no longer be assured without performing rather complicated encryption processing.
Against such a problem accompanying the encryption technique, there is an invention disclosed in, for example, JP-A-2002152191 and so forth as a radio communication method that pays attention to a physical feature of the propagation environment thereof. FIG. 23 shows the conventional radio communication system as described in the publication.
In FIG. 23, a propagation environment estimator 2311 in a transmitting station 2310 estimates the state information of a radio propagation channel 2330 shared only between the transmitting station 2310 and a receiving station 2320 which is the destination of transmitting data including confidential information. Then the transmitting station 2310 transmits the data including confidential information in view of this radio propagation environment. Due to this, because other radio stations having different radio propagation path environments cannot receive or reconstruct confidential information, the transmitting station can transmit confidential information with a high security.
However, in a broadband radio communication in general, due to its enhanced transmission rate, propagation parameters characterizing the propagation path, directivity, and polarization of antennas come to have frequency characteristic. Consequently, such a radio communication method as disclosed in the patent publication, wherein the transmitting station controls the propagation parameters by using a plurality of antennas, is available on the premise that the propagation parameters should be controlled within a particular frequency band, that is to say, within the range where the frequency characteristic of the antennas and the propagation paths are deemed to be uniform.
What is the problem to be solved is that, in the case of a broadband radio communication, characteristics of propagation paths and antennas can not be effectively utilized enough.